1. Field of the Invention
The present invention relates to media access control (MAC) in a wireless network, and more particularly, to a MAC method and system, which can vary a super-frame length in accordance with a channel status and a data characteristic in a wireless local area network (WLAN).
2. Description of the Related Art
A WLAN is a data communications network in which MAC layers and physical layers wirelessly operate a local area network. A plurality of mobile stations connected to an access point (AP) in the WLAN share a channel provided through the AP. In order to allow a plurality of mobile stations to access the shared channel with maximum performance, the MAC layers for the WLAN are standardized according to the IEEE (Institute of Electrical and Electronics Engineers) 802.11 standard.
A super-frame, defined in the IEEE 802.11 standard, represents an access to a wireless medium in a time axis. The wireless medium is a channel, having physical radio frequency (RF) characteristics. The super-frame comprises a distributed coordination function (DCF) period using a contention based CDMA/CA (Code Division Multiple Access/Collision Avoidance) protocol, and a point coordination function (PCF) period using a polling list to transmit data within a predetermined time period called a contention period. In other words, as shown in FIG. 1, the super-frame comprises a contention free period (CFP), i.e., the PCF period, in which frames or protocol data units can be transmitted without contention, and a contention period (CP), i.e., the DCF period, in which the frames or protocol data units can be transmitted with contention.
The frames transmitted during the CFP may be characterized as periodic data. For example, in wireless transmissions to a television, data continued in a constant unit is transmitted during the CFP. Therefore, during the CFP, the AP polls a selected mobile station to transmit the frames. On the other hand, the frames transmitted during the CP may be characterized as non-periodic data. For example, data transmitted through the Internet is transmitted during the CP. Therefore, during the CP, priority is assigned to the data frames from each mobile station, and the data frame assigned the highest priority during the CP has the highest chance to access a channel. The lengths of the CFP and the CP vary depending on the number of mobile stations connected to the AP, and the data characteristics.
However, according to the IEEE 802.11 MAC standard, the super-frame length is fixed according to the number of beacon frames, when a mobile station is initialized and when a basic service set (BSS) is produced, and, thus, the super-frame length is not changeable before (until) the BSS is eliminated (terminated) at which time the mobile station may be re-initialized with a new super-frame length according to the beacon frame numbers.
Since the super-frame length is fixed as described above, any variable ranges of the lengths of the CFP and the CP are limited to the super-frame length. Therefore, in a case where a payload of a frame to be transmitted is long, and the remaining super-frame duration is insufficient, the frame cannot be entirely transmitted. Thus, the corresponding data frame is fragmented at the transmitting stage so that a portion of the fragmented frame data can be transmitted during the corresponding super-frame period and then the remaining portion during the next super-frame period. When the data frame is transmitted fragmented as described above, the received fragments of the data frame must be de-fragmented at the receiving stage.
As described above, according to the conventional IEEE 802.11 MAC method, since the frame data is transmitted through fragmentation and de-fragmentation, it takes a long time to process the frame data in a WLAN. In addition, since the super-frame length is fixed, and, accordingly, a limited length (transmission duration) is available for burst data, the WLAN system capability for processing periodic data is not flexible. Since the WLAN system capability for processing periodic data is limited due to the super-frame, the entire WLAN system performance deteriorates.